2 Peter 2:22

it is happened unto them according to the true proverb Literally, that (saying) of the true proverb has happened to them … In the words that follow we have another of St Peter's references, without a formal citation, to the Book of Proverbs (Proverbs 26:11). See notes on 1Pe 4:8; 1 Peter 5:5. The form in which he gives the proverbs is participial. "The dog returned to his own vomit; the washed sow to her wallowing in the mire." We have, however, the colloquial, allusive form which the proverb had assumed in common speech rather than an actual quotation, and the second part of the proverb is not found in the passage referred to. In both cases stress is laid on the fact that there had been a real change. The dog had ejected what was foul; the sow had washed herself, but the old nature returned in both cases. Those who after their baptism returned to the impurities they had renounced, were, in the Apostle's eyes, no better than the unclean beasts. In the union of the two types of baseness we may, perhaps, trace a reminiscence of our Lord's teaching in Matthew 7:6.